Geographical boundary based tracking, some times referred to as geofence tracking, is the monitoring of movement of assets such as packages, equipment, vehicles, moving targets and personnel with respect to one or more defined geographic boundaries, regions or areas. The boundary may be imaginary (i.e., without fixed real world fences or boarders) or may be inherently defined (e.g., within the city limits or on a highway). Such tracking may be used to monitor and record the crossing, entry or exit of an asset (such as a vehicle) as it passes the geographical boundary and to alert a system operator of movement activities of the asset if the asset strays from a normal protocol.
A geographical boundary may define and enclose an area (e.g., defined by a circle), a corridor (e.g., defined by a pair of parallel lines), a boarder (e.g., defined by a single line) and the like. Initially, an asset may have an unknown position. Once a position is determined, the position along with its uncertainty may be inside the boundary, outside the boundary or straddling the boundary. An asset straddles a boundary when its area of position uncertain partially overlaps with both sides of the geographical boundary. An asset's location may be established by one or more position fixes. A position fix may be obtained via a single or a combination of a variety of modes or positioning sources, including but not limited to: stand-alone GPS with no system assistance; MS-based (Mobile Station-based) GPS with system assistance for initialization; MS-assisted (Mobile Station-assisted) with an external entity performing the fix; AFLT (Advanced Forward Link Trilateration) based on CDMA sectors triangulation; hybrid based on GPS and CDMA sectors triangulation; and sector center based on sector location. One skilled in the art would understand that other modes as well as various steps for processing the position fix may be used without altering the scope of the disclosure.
Position fixes have various confidence levels due to inherent errors. These inherent errors can greatly affect the reliability of geographical boundary based tracking. If the error level of the position fixes is high, the reliability of such tracking is reduced. The reliability of geographical boundary based tracking may also be reduced when only non-continuous position fixes are available. The level of reliability may depend on the time interval with which position fixes of the asset become available. Additionally, the reliability of geographical tracking may be affected by other conditions, such as the shape of the defined geographical boundary.